This invention relates to a resin composition affording a hardened surface on which a clear mark, sign, letter or the like pattern can be marked with a laser beam. The present invention is also directed to a coloring material for use in preparing the above composition and to a laser beam marking method.
There is a known marking method in which a laser beam is irradiated on a surface of a shaped body containing a laser marking material, so that the irradiated portions are colored or discolored to form a desired, discriminative pattern on the surface of the shaped body. Such a laser marking material, e.g. a lead compound, is mixed in a resin matrix material and the resulting composition is shaped into a desired form. The known composition, however, has a problem because a clear, high contrast pattern is not obtainable even if the irradiation is sufficiently carried out.
To cope with this problem, JP-A-4-28758 and JP-A-4-183743 propose a laser beam absorbing resin composition including a colorant capable of discoloring upon being irradiated with a laser beam, a laser bean absorbing substance selected from calcium pyrophosphate, triphenyl phosphine, calcium hexafluorosilicate and zirconium silicate, and an epoxy resin. Because of the presence of the laser beam absorbing substance, the absorption of the laser beam is enhanced so that the coloring reaction of the colorant is accelerated. The known technique is, however, still unsatisfactory in forming a clear, high contrast pattern.